Botulinum neurotoxins (BoNTs) are extremely potent proteins with a mouse lethal dose of 0.3 ng/kg. Seven serotypes (A-G) of BoNTs are produced by different strains of Clostridium botulinum. 
The mechanism by which BoNTs cause botulism has been well studied. After ingestion, the neurotoxin is translocated across the intestinal mucosa, gaining access to neuromuscular junctions. At affected junctions, the neurotoxin is internalized by neurons via endocytosis. Inside the cell, the protease activity of the toxin degrades specific vesicular and plasma membrane proteins, disrupting neurotransmitter release from the neuron. Thus, the patient experiences paralysis due to an inability to release neurotransmitters from the presynaptic surface.